The present invention relates to spectrometers which incorporate interferometers, and particularly to improved apparatus and methods for adjusting the optical elements to produce the desired alignment of the radiation beams, particularly the analytical beam.
Spectrometers of the type under discussion, which incorporate Michelson interferometers, have a fixed-length arm and a variable-length arm which is varied to cause spectral scanning. They generally incorporate three separate beams, each of which is affected by the scanning motion of the reflector in the variable-length arm. An analytical beam, which preferably consists of infrared radiation, is a relatively large beam, which is focused on the sample, in the sample region, to provide repetitious interferograms which are integrated and converted electronically to the desired read-out data. A clock beam, which is preferably a monochromatic (laser) beam, is used to determine the points at which readings are taken during each scanning motion. A reference beam, which is preferably a wide band (white light, and possibly infrared) beam provides interferograms which are used as the synchronizing means for starting each analytical scan at the same point, while offsetting the peak of the analytical scan from the peak of the reference scan (which is the convenient scan-initiating point).
Proper alignment of a Michelson interferometer requires that several alignment conditions be met:
(a) The mirrors which define the two arms of the interferometer must be properly aligned with respect to the beamsplitter so as to superimpose the two reflected beams and produce a coherent interferogram. PA0 (b) The analytical beam should be perpendicular to the scanning mirror so as to avoid wavefront shear (and hence loss of coherence) during scanning. In the case of non-perpendicularity, the curved wavefronts will become non-parallel as the interferometer is scanned away from the equal-path-length condition. PA0 (c) The laser clock beam should be perpendicular to the scanning mirror so as to insure a continuous clock signal throughout the scanning range. PA0 (d) The clock beam and the analytical beam should be parallel to one another so as to provide a frequency scale which is accurate and reliable from instrument to instrument, or after realignment of a given instrument. PA0 1. Using the laser, which is in the instrument to provide clock signals, as the source of radiation for the alignment process; PA0 2. Directing the laser radiation through a target both on its way from the laser generator to the interferometer, and as it is reflected back from the interferometer, thereby validating the perpendicularity of each reflector to the laser beam; PA0 3. Using a semi-transparent mirror, i.e., a small beamsplitter, in the path of the laser beam ahead of the interferometer beamsplitter, thereby enabling portions of the laser beam to go into both the interferometer and the sample chamber; and PA0 4. Using a second semi-transparent mirror (beamsplitter) in the path of the laser beam, for the purpose of providing two parallel laser beams which: (a) permit efficient checking of the optical elements in the sample chamber; (b) permit the use of an aperture in the sample region to select, for use as the analytical beam, that portion of the analytical signal which is parallel to the laser beam in the interferometer; (c) provide tracking means for installing accessories and samples in the sample chamber; and (d) make available a separate laser beam for each of the side-by-side mirrors in those systems where two mirrors, offset in position from one another, are used to reflect the analytical and synchronizing (reference) beams, as disclosed in Doyle Application Ser. No. 470,937, filed Mar. 1, 1983.
The present invention is concerned primarily with the use of the laser beam, already included in the system, as the means for providing an additional significant function, i.e., the convenient adjustment of the optical elements for alignment purposes. Laser beams have been used for optical alignment purposes, but usually by bringing a laser device temporarily into the system for alignment only. Or, in some instances, alignment has been accomplished solely by adjusting the optical element to provide the most satisfactory interferometer output.
Previous alignment strategies have been deficient in at least two respects. They have been undesirably time-consuming, and they have failed to insure the highest degree of accuracy in their results.